1. Field of the Invention
The present invention relates to a novel method of bonding a cation exchange membrane of a fluorinated polymer. More particularly, it relates to a novel method of bonding a cation exchange membrane of a fluorinated polymer for an electrolysis which is used for an electrolysis of an aqueous solution of an alkali metal chloride to produce chlorine and an alkali metal hydroxide.
2. Description of the Prior Art
It has been known to produce chlorine in an anode compartment and alkali metal hydroxide in a cathode compartment by partitioning the anode compartment and the cathode compartment with a diaphragm and feeding an aqueous solution of an alkali metal chloride into the anode compartment to electrolyze it. The method has been known as a two-compartment type diaphragm electrolysis.
Thus, it has been proposed to use a cation exchange membrane of fluorinated polymer which does not substantially pass the electrolyte and selectively pass only alkali metal ions and has high alkali resistance and chlorine resistance, to produce an alkali metal hydroxide which does not substantially contain an alkali metal chloride as the ion exchange membrane electrolysis.
It has been found that the cation exchange membranes of the carboxylic acid type fluorinated polymer are superior to cation exchange membranes of the sulfonic acid type fluorinated polymer in electrolytic characteristics.
Thus, an alkali metal hydroxide having a high concentration such as 25 to 45 wt.% can be produced at higher than 90% of a current efficiency by using the cation exchange membrane of the carboxylic acid type fluorinated polymer, and the membrane has rather less electric resistance in comparison with the cation exchange membrane of the sulfonic acid type fluorinated polymer.
On the other hand, when these cation exchange membranes are used for various purposes, it is sometimes required to bond the cation exchange membrane by various reasons, though it is not restricted to the cation exchange membrane of the carboxylic acid type fluorinated polymer which is used for the electrolysis of an aqueous solution of an alkali metal chloride.
For example, it is necessary to bond the cation exchange membrane for repairing the membrane when pin-holes, peelings or breaking at a part of the membrane is caused. It is also necessary to bond the membranes to form a specific shape such a bag shape by bonding two or more membranes or to fix the membrane on the other synthetic resin in one piece.
In the bonding operation, it is necessary to melt the membrane enough to give a viscosity required for bonding the membrane at the bonding part, except using adhesives.
The inventors have studied the bonding of cation exchange membranes of fluorinated polymers having carboxylic acid type exchange groups and have discovered that the melting properties of the membrane depend upon the kind of carboxylic acid type ion exchange group on the membrane.
For example, when the ion exchange groups are in the form of the alkali metal salt of carboxylic acid which is the form of the ion exchange groups in the electrolysis of the aqueous solution of an alkali metal chloride, the polymer of the bonding part is decomposed before or during melting it even though the bonding part is melted under applying high pressure. Thus the membrane could not be bonded in this case.
When the ion exchange groups of the membrane are in the form of --(COO).sub.m X wherein X represents an alkali metal or an alkaline earth metal or HNR.sub.1 R.sub.2 R.sub.3 and R.sub.1 to R'.sub.3 represent hydrogen or a lower alkyl group and m is a valence of X, the membrane is decomposed before or during melting it whereby it is impossible to bond the membrane.